This invention relates to transfer cases for four-wheel drive systems, and more particularly to a transfer case incorporating a viscous coupling and a gear reduction planetary assembly.
Four-wheel drive systems have been in existence for many decades. The output of the engine has been conventionally split between the front wheels and the rear wheels by a transfer case. One type of system used for many years provided undifferentiated power to all wheels of the vehicle. This gave the vehicle good mobility under adverse surface conditions. However, this type of drive was not particularly useful for normal highway driving in that, because all four wheels operated at the same speed, the vehicle could not be turned without sliding or scuffing one or more tires. This drawback resulted in the development of systems which included a conventional two-wheel drive mechanism. For highway use the two-wheel drive was utilized and this caused the vehicle to assume the characteristics of most other two-wheel drive vehicles.
It is desirable in a four-wheel drive vehicle to obtain the benefits of differentiation provided in conventional two-wheel drive vehicles while also obtaining the benefits of a conventional four-wheel drive, when needed. In these circumstances, transfer cases were developed that were manually actuated to lock out a differential unit when the differentiation was not desired. However, the procedures of manually locking out a differential unit are not practical in that the user must stop the vehicle prior to engaging the all-wheel drive system. Accordingly, efforts have been made to replace the manual lock-out type operation by an automatic locking apparatus. One type of automatically operated apparatus developed utilizes electronic sensing apparatus which senses a difference in rotation speeds of two rotatable speeds of two rotatable parts of the differential. When the difference in speeds exceeds a predetermined value, a solenoid is actuated to operate a mechanical clutching mechanism which couples the two rotatable parts together and renders the differential ineffective.
Another type of automatic apparatus includes a viscous coupling having a plurality of plates connected to one rotatable member and interleaved with a plurality of plates connected to another rotatable member. To this end, viscous couplings have been used in numerous power transmission applications such as four-wheel drive transfer cases, differentials and limited-slip intra-axle and inter-axle devices. In such couplings, viscous fluid substantially fills the housing containing the interleaved plates. As the speed difference between two rotating parts increases, the viscous fluid is sheared by the interleaved elements, which results in a tendency to interlock the rotatable parts. Examples of transfer cases which incorporate a viscous coupling therein are disclosed in U.S. Pat. No. 4,031,780 to Dolan et al., U.S. Pat. No. 5,046,998 to Frost, and in U.S. Pat. No. 5,078,660 to Williams et al., all commonly assigned to the assignee of the instant application.
In the patent to Dolan et al., a transfer case for a four-wheel drive train is disclosed that includes a differential incorporating both a viscous coupling and a viscous fluid operated mechanical clutch which may be operated to inhibit and prevent differentiation.
In the patent to Frost, a transfer case for a four-wheel drive vehicle is disclosed that provides a drive range planetary gear set that is selectively positionable between low and high speed output positions and a neutral position. A dual planetary inter-axle differential gear set is axially spaced from the range planetary gear set. The latter gear set may be positioned to provide either a two-wheel drive mode to a first output or a full-time four-wheel drive mode with differential action between the first output and a second output. A viscous fluid clutch is connected between the first and second outputs for modifying the torque division between these outputs.
Finally, in the patent to Williams et al., a transfer case for full-time four-wheel drive vehicles is disclosed having limited slip between the front and rear drive lines. Like the transfer case of the Frost patent, the transfer case of the Williams et al. patent also includes a drive range planetary gear set that is selectively positionable between low and high drive ranges and a neutral position. A viscous coupling chamber is provided in conjunction with a dual planetary differential gear set for providing full-time four-wheel drive differentiation with limited slip between first and second outputs.
The present invention relates to an transfer case which includes both mechanical and viscous coupling features. The present invention provides a transfer case having operating characteristics and advantages different from those of the prior art.